Rheological Techniques as a Tool To Analyze PolymerBitumen

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Rheological Techniques as a Tool To Analyze Polymer-Bitumen Interactions: Bitumen Modified with Polyethylene and Polyethylene-Based Blends O. Gonza´lez,† J. J. Pen˜a,‡ M. E. Mun˜oz,† A. Santamarı´a,*,† A. Pe´rez-Lepe,§ F. Martı´nez-Boza,§ and C. Gallegos§ Departamento de Ciencia y Tecnologı´a de Polı´meros, Facultad de Quı´mica, Universidad del Paı´s Vasco, P.O. Box 1072, E-20080 San Sebastia´ n, Spain, Departamento de Fı´sica de Materiales, Facultad de Quı´mica, Universidad del Paı´s Vasco, P.O. Box 1072, E-20080 San Sebastia´ n, Spain, and Departamento de Ingenierı´a Quı´mica, Escuela Polite´ cnica Superior, Universidad de Huelva, Ctra de Palos de la Frontera s/n 21819, Huelva, Spain Received February 25, 2002

We analyze the rheology of a bitumen modified with 5% of high- and low-density polyethylene and their respective blends with ethylene/propylene/ethylidene norbornene. A broad mechanical relaxation, observed around 30 °C, is associated with the collapse of a compact structure, constituted by asphaltene particles surrounded by solid resin. The addition of polymer to bitumen does not modify this relaxation. Above 30 °C the dynamic functions decrease as: HDPE/bitumen > HDPE-EPDM/bitumen > LDPE-EPDM/bitumen > EPDM/bitumen > LDPE/bitumen > bitumen. We assume that the degree of polymer-bitumen interaction, follows the same order. This is in apparent contradiction with the results one could expect from solubility considerations.

Introduction Nowadays a very large majority of the roads are constructed using a mixture of bitumen (5 wt %) and mineral aggregates. Notwithstanding this low bitumen content, the performance of the road pavement depends to a large extent on the properties of bitumen itself, since it constitutes the only deformable component. The correlation between the complex colloidal structure of bitumen and its viscoelastic response is therefore a subject of scientific and technical interest. Among the abundant literature devoted to this subject, we dare to recommend the paper by Lesueur et al.,1 which besides proposing a sound model for bitumen viscoelasticity, gives a general overview of the different theoretical approaches. On the other hand, from a technological point of view, the increasing traffic load has resulted in a corresponding continuous improvement of bitumen grades. However, the limit of increasing pavement performance with conventional pure bitumen seems to have been reached. The addition of synthetic polymers to enhance service properties over a wide range of temperatures in road paving applications was considered a long time ago and nowadays has become a real alternative. As has been pointed out in relevant papers about polymer/bitumen blends,2,3 understanding the interactions of asphaltene and maltene (main compo* Corresponding author. Telephone: +34 943 018184. Fax: +34 943 212236. E-mail: [email protected]. † Departamento de Ciencia y Tecnologı´a de Polı´meros, Facultad de Quı´mica, Universidad del Paı´s Vasco. ‡ Departamento de Fı´sica de Materiales, Facultad de Quı´mica, Universidad del Paı´s Vasco. § Universidad de Huelva. (1) Lesueur, D.; Gerard, J.; Claudy, P.; Le´toffe´, J. J. Rheol. 1996, 40, 813-836.

nents of bitumen) with the polymer, is a crucial point to gain insight into the routes to improve the capacities of these systems. Bitumen/polymer proportion varies in function of application, for instance typically 5 wt % polymer for road paving and 30 wt % for roofing membranes. Homopolymers, like high- and low-density polyethylenes and polypropylene, as well as random and block copolymers, like ethylene-vinyl acetate, ethylene/ propylene, styrene-b-butadiene-b-styrene, and styreneb-ethylene-co-butylene-b-styrene, have been used as bitumen modifiers. In the particular case of polyethylenes, several papers and patents4-11 have deepened on the improvement of practical aspects such as cracking at low temperatures, permanent deformation (“rutting”), fatigue strength, and others. In one of these papers4 the effect of adding polyethylene to the bitumen is compared with that of adding a blend of polyethylene and ethylene-propylene-diene terpolymer. The use of recycled polyethylenes has also been recently envisaged.12,13 (2) Ho, R.; Adedeji, A.; Giles, D. W.; Hajduk, D. A.; Macosco, C. W.; Bates, S. F. J. Polym. Sci. B: Polym. Phys. 1997, 35, 2857-2877. (3) Wloczysiak, P.; Vidal, A.; Papirer, E.; Gauvin, P. J. Appl. Polym. Sci. 1997, 65, 1595-1607. (4) Ait-Kadi, A.; Brahimi, H.; Bousmina, M. Polym. Eng. Sci. 1996, 36, 1707-1723. (5) Fawcett, A. H.; McNally, T.; McNally, G. M.; Andrews, F.; Clarke. J. Polymer 1999, 40, 6337-6349. (6) Newman, J. K. J. Elastom. Plast. 1998, 30, 245-263. (7) Hemersam, R. U.S. Patent 4,240 946, Bunzl & Biach Aktiengesellschaft, 1980. (8) Batiuk, M., et al. U.S. Patent 3,941,859, The B. F. Goodrich Co., 1976. (9) Ho, R., et al. U.S. Patent 5,302,638, Husky Oil Operations Ltd., 1994. (10) Iacono, C.; Tribastone, S. European Patent 315239, Enichem Anic Spa., 1989. (11) Thompson, A. G.B. Patent 2,219,802, Vulcanite Ltd., 1989.

10.1021/ef020049l CCC: $22.00 © 2002 American Chemical Society Published on Web 07/16/2002

Analyzing Polymer-Bitumen Interactions

In this paper we analyze the viscoelasticity of a bitumen modified with 5% of high- and low-density polyethylene and their respective blends with ethylene/ propylene/ethylidene norbornene. The principal objective is to gain insight into the correlation between the structure and viscoelasticity of bitumen itself and to disclose the rheological effects of the interaction of the polymers with asphaltene and maltene. The paper is organized following these steps: ‚Characterization of polyethylene-based blends: the degree of miscibility may have a certain repercussion on the properties of polymer/bitumen blends. ‚Analysis of the solid-state mechanical relaxations of pure bitumen and polymer/bitumen blends. This provides valuable information on the structure of bitumen and the effect of polymer incorporation. ‚Oscillatory flow measurements in the temperature range 30-70 °C, that is above the main mechanical relaxation. Some considerations are made on the effect of polymer/liquid resins interactions. ‚Oscillatory flow measurements at 10 °C, that is below the main mechanical relaxation. Besides solubility (interaction) factors, the effect of the rigidity of the polymer must be considered to have a general interpretation of the results.

Energy & Fuels, Vol. 16, No. 5, 2002 1257 Solid-state dynamic viscoelastic measurements of all the samples (pure polymers, EPDM/PE polymer blends, pure bitumen, and polymer/bitumen blends) were accomplished in a Polymer Laboratories Mark I dynamic mechanical analyzer (DMTA). The experiments were carried out in bending mode at a heating rate of 4 °C/min (starting at -150 °C) and a frequency of 1 Hz (6.28 rad/s). Continuous flow and oscillatory flow measurements of pure polymers and EPDM/PE blends in the molten state (T ) 180 °C) were performed in plate-plate mode (diameter 2.5 cm) in a Rheometric ARES equipment. The rheological study of polymer/bitumen blends in the temperature range 5-100 °C was realized in plate-plate mode (diameter 2 cm) in a stresscontrolled TA Instrument CSL 100. With both types of equipment oscillatory flow data were obtained in the linear viscoelastic regime. The same protocol, which lies in pouring molten products at mixing temperature (180 °C) in molds adapted to the dimensions of the measuring plates, was used for sample preparation. Repeatable results were obtained with at least two different samples of each product. In our study, the comparison is done with pure bitumen submitted to the same thermomechanical history as polymer/ bitumen blends, i.e., under the same conditions as the blends, but without adding the polymer.

Results and Discussion

All the blends were based on the same bitumen, a 60/70 penetration bitumen from Repsol-YPF. Some characteristics of this bitumen, determined using ASTM tests, are: asphaltene content 24.5 wt %; penetration 25 °C (1/10 mm) 67; softening point (ring and ball) 48.2 °C; penetration index -0.9, and Fraass breaking point -8 °C. Three polymers were mixed with bitumen: a high-density polyethylene, HDPE 6006-L (Mw ) 171500, Mw/Mn ) 7.7, and density ) 0.956 g/cm3), supplied by Repsol-YPF; a low-density polyethylene, LDPE 302R (density ) 0.924 g/cm3), supplied by Dow Chemicals, and an ethylene-propylene-diene terpolymer, EPDM Buna EPG 8450 (Mw ) 330000, Mw/Mn ) 2.06, ethylene/propylene/ ethylidene norbornene ratio of 54/42/4), supplied by Bayer. Respective blends of EPDM with HDPE and LDPE in a 25(EPDM)/75Polyethylene(PE) proportion were also used to modify pure bitumen. These polymer blends were prepared using a twin screw Collin ZK at a rotating speed of 50 rpm and a temperature of 160 °C. The blends were processed two times under the same conditions to attain homogeneous materials. Test specimens were obtained in a compressionmolding machine at 160 °C under a pressure of 15 MPa during 5 min. Polymer/bitumen blends were prepared by mechanical stirring. The corresponding polymer (or polymer blend) was added to bitumen at a level of 5 wt % by weight and mixing was maintained at 180 °C and 1200 rpm for 6 h. After mixing, the resulting dispersion was poured into a small can and then stored in a freezer (T ) -25 °C) to retain the obtained morphology. The rate of heating and freezing were, respectively, 2 °C/min and 10 °C/min. In real conditions of road application the main problem in the preparation and use of polymer-modified bitumens is the stability of the product. A rheological analysis of our blends, based on the method described by Yousefi et al.,13 shows that these are stable for 6 h at a temperature of 180 °C.

The degree of homogeneity, what we can call the compatibility, of EPDM/HDPE and EPDM/LDPE blends is analyzed, in solid state by DMTA and in melt state by steady continuous flow and dynamic viscoelastic measurements. Mechanical relaxations in solid state are analyzed by loss tangent, tan δ, plots against temperature, as those shown in Figure 1. The loss peaks detected for pure EPDM reflect the existence of a relaxation at -36 °C, associated to the glass transition temperature, Tg. A secondary γ relaxation has been reported for this copolymer around -110 °C,14 although the scattering and gathering of data of Figure 1 does not allow a clear recognition. On the other hand, it is known15 that depending on the molecular architecture, polyethylenes can show three relaxations: R, β, and γ, respectively at (-125 to -110 °C), (-25 to -10 °C), and (50-100 °C). It is assumed that the glass transition temperature corresponds to γ relaxation, whereas R relaxation is assigned to the crystalline phase and depends, hence, on the chain characteristics of each polyethylene, HDPE, LDPE, or linear low-density polyethylene (LLDPE). The degree and length of branching is usually reflected in the magnitude of the β relaxation peak, a relaxation which consequently is not detected in HDPE samples. Deeming literature results and our own data of Figure 1 for pure polymers, we discard the use of γ transition for miscibility analysis. Instead we consider the glass transition temperature of EPDM and β relaxation of LDPE. We observe that for both blends Tg of EPDM is slightly shifted to a lower temperature (from -36 to -41 °C), which would indicate a small plasticizing effect of amorphous polyethylene phase, since the latter has a lower Tg than EPDM. β relaxation is only observed (as a shoulder at -15 °C) in LDPE and its blend (Figure 1b). For EPDM/LDPE blends, it has been reported16 that as LDPE content is increased, the

(12) Durrie, F.; Migliori, F.; Platret, G. Bull. Lab. Ponts Chaussees. 1999, 221, 3-12. (13) Yousefi, A. A.; Ait-Kadi, A.; Roy, C. J. Mater. Civil Eng. 2000, 12, 113-123.

(14) Kalfoglou, N. K. J. Macromol. Sci. Phys. 1983, B22, 343-362. (15) Lee, H.; Cho, K.; Ahn, T.; Choe, S.; Kim, I. J.; Park, I.; Lee, B. H. J. Polym. Sci. 1997, 35, 1633-1642. (16) Starkweather, H. W. J. Appl. Polym. Sci. 1980, 25, 139-147.

Experimental Section

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Figure 2. Steady-state viscosity as a function of shear rate for pure polymers and their blends at T ) 180 °C, as determined in plate-plate continuous flow.

Figure 1. Mechanical loss tangent as a function of temperature for pure polymers and their blends. The figure is divided in two parts (a and b) to overstress the difference between EPDM/HDPE and EPDM/LDPE blends. The arrow in Figure 1b indicates β relaxation of LDPE (see text).

Tg of EPDM shifts toward the β relaxation of the LDPE, a result which cannot be confirmed in our case. However, the existence of the β relaxation and the associated increase of losses between -40 and 50 °C (with respect to EPDM/HDPE blend) is relevant because it marks the difference between both blends. The higher losses found in this temperature range may be associated with an interaction between the phases, which would produce a stronger interfacial adhesion in the EPDM/LDPE blend. No chemical bonding between polyethylene matrix and EPDM is in any case expected, according to DSC results reported in the literature.17 Rheological results of pure polymers and their blends in the molten state, are shown in Figures 2 to 4. In all the considered shear rate range, the viscosity of EPDM is several times larger than that of both HDPE and LDPE samples, so as at mixing temperature (180 °C, the same as rheological measurements) polyethylene should tend to form the continuous phase or matrix. As can be seen in Figures 2 and 3 the viscosity of the EPDM/LDPE blend is considerably higher than that of EPDM/HDPE, notwithstanding the viscosity of both original polyethylenes is similar. This is a significant feature to distinguish both blends. Actually this result may be associated with the increase of losses between (17) Yousefi, A. A.; Ait-kadi, A. Adv. Polym. Technol. 1998, 17, 127143.

Figure 3. Steady-state viscosity function η(γ˘ ) and oscillatory flow complex viscosity function η*(ω) (Cox-Merz plots) for polymer blends. The circles and line correspond to the EPDM/ HDPE blend and the squares and broken line to that of EPDM/ LDPE. (see text).

-40 and 50 °C observed in EPDM/LDPE with respect to EPDM/HDPE blend, since this leads to an increase in interfacial adhesion, which in turn provokes a viscosity enhancement. Viscosity function η(γ˘ ) and complex viscosity η*(ω) results are compared in Figure 3 using the well-known Cox-Merz empirical rule.18 As could be expected, this rule does not hold for these two-phase systems, but the plots confirm the viscosity increase also for oscillatory measurements. In Figure 3 we include the viscosities that correspond to the simple additive rule, which marks a difference between values below (EPDM/HDPE) and above (EPDM/LDPE) this rule. The positive deviation of the viscosity data of the EPDM/ LDPE blend can be considered19 as a symptom of strong interaction between the phases, agreeable with the aforementioned enhanced interfacial adhesion. First normal stress N1 versus shear rate results, shown in Figure 4, also indicate an enhancement of melt elasticity of the EPDM/LDPE blend, which approaches that of pure EPDM, whereas the elasticity of the EPDM/HDPE blend hardly overcomes that of HDPE. (18) Cox, W. P.; Merz, E. H. J. Polym. Sci. 1958, 28, 619-622. (19) Han, C. D. Multiphase Flow in Polymer Processing; Academic Press: New York, 1981.

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Figure 4. First normal stress difference as a function of shear rate for pure polymers and their blends at T ) 180 °C, as determined in plate-plate continuous flow.

Figure 5. Mechanical loss tangent as a function of temperature for pure bitumen and three of the polymer/bitumen blends. The arrow marks a shoulder associated with the glass transition temperature of EPDM.

Therefore, solid-state DMTA results and molten state rheological results allow us to state that EPDM interacts more with LDPE than with HDPE. DMTA results of pure bitumen, as well as bitumen/ polymer blends, obtained in bending mode are presented in Figure 5. A broad peak, centered at 33 °C for bitumen itself, is observed in loss tangent, tan δ, plots against temperature. Recent DMTA measurements performed in our laboratory20 exhibit relaxations such as those displayed in Figure 5 for pure bitumens of different penetration degrees, with peaks ranging from 20 to 57 °C. To our knowledge such a relaxation has not been analyzed in the literature, probably because its detection depends on critical experimental conditions, as can be seen in Figure 6, where different measuring requirements are compared. It is observed that measurements in shear mode, and starting at 0 °C instead of -80 °C, give just a continuous increase of tan δ. Fawcett et al.5,21 report an unidentified maximum in loss modulus E′′ at 15 °C and a sharp increase of loss tangent at 20 °C for a 100 penetration grade bitumen, with no detectable maximum because the sample becomes too soft to (20) Barral, M.S. Thesis in progress. (21) Fawcett, A. H.; McNally, T. J. Polymer 2000, 41, 5315-5326.

Figure 6. Comparison of loss tangent results using two different experimental conditions: (Ο) bending mode; (∆) shear in plate-plate geometry mode. (a) refers to pure bitumen and (b) to EPDM-HDPE/bitumen blend. The rest of the blends give similar results.

further measurements in shear mode. Neither are very conclusive results presented by Wloczysiak et al.3 Although a transition at 20 °C is noticed from the peak of the loss modulus E′′ for a bitumen, this is not apparent in tan δ, because the storage modulus decreases continuously. No more clarifying are the DMTA results presented by Sabbagh and Lesser22 who report a practically continuous increase of loss tangent with temperature up to 40 °C, with a small peak of unknown tan δ value at 13 °C for an asphalt of 20 penetration grade. The authors identify this small relaxation as the glass transition temperature. But considering reported Tg values obtained by differential scanning calorimetry (DSC) measurements, which are in the range -11 to -25 °C,1 we discard to associate the observed relaxation to the glass transition of the bitumen. Actually, Lesueur et al. assume that the transition observed by DSC characterizes the Tg of the maltene matrix. On the other hand, using modulated differential scanning calorimetry (MDSC) Memon and Chollar23 and Masson and Polomark24 confirm the glass transition of maltenes and the (22) Sabbagh, A. B.; Lesser, A. J. Polym. Eng. Sci. 1998, 38, 707715. (23) Memon, G. M.; Chollar, B. H. J. Therm. Anal. 1997, 49, 601607. (24) Masson, J. F.; Polomark, G. M. Thermochim. Acta 2001, 374, 105-114.

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latter authors report another glass transition temperature at 70 °C for the asphaltenes. Therefore, deeming the complex nature of bitumen, which actually is a colloidal dispersion, the attempts to define a glass transition of the material itself become rather futile. Both aforementioned glass transitions of maltene and asphaltene (at approximately -20 and 70 °C, respectively) are practically in the range of the very broad DMTA relaxation which reflects the complex evolution of the colloidal dispersion with temperature. The dependence of the amount of total solid phase (asphaltene covered by solid resins) on temperature and the state of maltene matrix (which vitrifies below approximately -20 °C) are the key to understand the structurerheology relationship in bitumen.1,25 On the basis of the scheme proposed by Lesueur et al. (see Figure 7 of ref 1) we associate the broad tan δ relaxation of pure bitumen with the following phenomena: (a) The initial increase of tan δ, starting at approximately -30 °C, would correspond to the glass transition process from a vitrified to a liquid maltene matrix. (b) This process is overlapped by the transition from a compact structure, constituted by asphaltene particles surrounded by solid resins, to a particle-free dispersion. The change is activated by temperature, since solid resins tend to dissolve as temperature is increased, reducing the shell that surrounds the asphaltene. (c) The collapse of the compact structure causes the maximum in tan δ, which is experimentally detectable because, under experimental DMTA conditions, asphaltenes give the necessary strength to the sample. As can be seen in Figure 5, the relaxation is not practically modified when polymer is mixed with bitumen. According to the explanation we give for the relaxation observed in bitumen itself, we are impelled to assume that the compact structure constituted by asphaltenes at low temperature is not altered by the presence of a 5% polymer. This is a logical consequence of the fact that, considering the solubility parameter values presented below, the polymer probably interacts with saturates, whereas asphaltenes rather adsorb polar aromatics. These interactions have a very noticeable effect on rheological properties, especially at high temperatures, but apparently do not have repercussion on the relaxation associated with the collapse of asphaltene-based compact structure. On the other hand we remark the shoulder observed at -35 °C in EPDM/ bitumen blend, which very likely is due to the glass transition temperature of the copolymer (see Figure 1). This gives rise to improved tan δ values in the range -40 to -10 °C which reflects an increase of the flexibility, preventing cracking in cold weather. As can be seen in Figure 5, the other blends do not show such a potential advantage, because Tg of polyethylenes is much less noticeable. The same practical application of DMTA results has been reported by Fawcett et al. 21 for polymer/bitumen blends prepared for roofing. The effect of frequency on storage modulus G′ and complex viscosity η*, obtained in shear torsion mode in a 30 °C to 70 °C range, is displayed, respectively, in (25) Sawatzky, H.; Farnand, B.; Houde, J.; Clelland, I. Temperature dependence of complexation processes in asphalt and relevance to rheological temperature susceptibility. Proceedings of the ACS Symposium on Chemistry of Asphalt and Asphalt-Aggregates Mixes, Washington, DC, 1992; pp 1427-1436.

Gonza´ lez et al.

Figure 7. Storage modulus G′ master curves for all the samples, obtained using frequency-temperature superposition method and a reference temperature TR ) 30 °C. aT is the shift factor which accounts for the frequency-temperature equivalence.

Figure 8. Complex viscosity η* master curves for all the samples, obtained using frequency-temperature superposition method and a reference temperature TR ) 30 °C. aT is the shift factor which accounts for the frequency-temperature equivalence.

Figures 7 and 8. We first remark on the apparent frequency-temperature equivalence which allows us to obtain good G′ and η* master curves for all the systems. However, no sound conclusion must be drawn on the fact that master curves are obtained, since it is proved that bitumen itself is a complex material. Time or frequency-temperature superposition leading to a single master curve, what is called “thermorheological simplicity” of polymer systems, has been the subject of many apparent contradictions, some of which have been disclosed by Lesueur et al.1,26 for the particular case of pure bitumen and polymer/bitumen blends. The use of different procedures or rheological plots27-30 to analyze the thermorheological complexity may lead to different conclusions. These previous findings have lead us to use (26) Lesueur, D.; Gerard, J. F.; Claudy, P.; Le´toffe´, J. M.; Martin, D.; Planche, J. P. J. Rheol. 1998, 42, 1059-1074. (27) Han, C. D.; Lem, K. Polym. Eng. Rev. 1982, 2, 135-165. (28) Harrell, E. R.; Nakajima, N. J. Appl. Polym. Sci. 1984, 29, 9951010. (29) Mavridis, H.; Shroff, R. N. Polym. Eng. Sci. 1992, 32, 17781791. (30) Za´rraga, A.; Pen˜a, J. J.; Mun˜oz, M. E.; Santamarı´a, A. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 469-477.

Analyzing Polymer-Bitumen Interactions

Figure 9. Breakup of frequency-temperature superposition for bitumen itself, as observed in phase angle δ versus complex modulus plot. (See text).

plots of phase angle δ versus complex modulus G* to analyze our samples: the break of time-temperature superposition or thermorheological complexity of bitumen itself is clearly demonstrated in Figure 9. As can be observed in Figures 7 and 8 both dynamic viscoelastic functions G′ and η* increase when a polymer is added to the bitumen. At the considered temperatures, G′′ > G′ for all the samples (not shown in Figure 8), which indicates a liquid material response, although, as could be expected, the scaling laws (G′ ∝ ω2 and G′′ ∝ ω) established for homogeneous polymeric liquids31 are not fulfilled. The most dramatic enhancement of the viscoelastic functions (more than 20 times at low frequencies) is found when HDPE is added to the bitumen, notwithstanding HDPE itself is not the most viscous polymer (see Figures 2-3) This enormous effect is little altered when EPDM is included in the blend to form the ternary HDPE-EPDM/bitumen blend. Surprisingly enough we observe that, whereas the results of HDPE-EPDM/bitumen blends closely approach those of HDPE/bitumen blends, the results of LDPE-EPDM/ bitumen blends are very similar to those of EPDM/ bitumen blends. That is to say that in the former case HDPE plays the dominant role, while in the latter EPDM behavior is imposed. From DMTA and molten rheological results of polyethylene/EPDM blends (Figures 1-4), an interaction between LDPE and EPDM is envisaged. On the contrary, it is concluded that EPDM does not interact with HDPE, so the role of the latter is just to cover EPDM. Therefore, to the difference of LDPE-EPDM blend, HDPE-EPDM appears face to bitumen like pure HDPE, justifying the similar results for HDPE/bitumen and HDPE-EPDM/bitumen observed in Figures 7 and 8. On the other hand, we point out that at 30 to 70 °C range, the modulus and viscosity ranking of our blends (HDPE/bitumen > HDPE-EPDM/ bitumen > LDPE-EPDM/bitumen > EPDM/bitumen > LDPE/bitumen) is in contradiction with the findings of Yousefi et al.13 for polyolefines/bitumen blends. These authors found a higher dynamic viscosity η′ and complex modulus G* for EPDM/Bitumen than for HDPE/Bitumen, allegedly because polypropylene (PP) sequences of EPDM interact more than other polyolefines with bitu(31) Bird, R. B.; Armstrong, R. C.; Hassager, O. Dynamics of Polymeric Liquids, Vol. 1; Wiley: New York, 1987.

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men. The solubility parameter window of bitumen29,32 (asphaltenes: 25-33 (MPa)0.5; polar aromatic resins: 21.9-26.6 (MPa)0.5; naphthene aromatics: 19-22.5 (MPa)0.5; and saturates: 17.4-20 (MPa)0.5); as compared with the solubility parameter of PP and our polymers33,34 (PP: 16.8-18.8 (MPa)0.5; PE: 15.8-17.1 (MPa)0.5; and EPDM: 16.4 (MPa)0.5); indicates that polymers should interact mostly with saturate resins, owing to their closer solubility. From a thermodynamical point of view PP appears as slightly more interacting, but other factors such as rheology of polymers during mixing can also play an important role. As we have pointed out in a recently prepared paper,35 this latter aspect is very relevant, because the polymer dispersion morphology determines polymer-bitumen interaction in the molten state. Considering that we use a different mixing procedure and a different grade of bitumen compared to Yousefi et al.,13 the aforementioned contradictory results should not be surprising. Unfortunately, we are not able to reproduce mixing conditions (shear and elongational rates, actual temperature, etc.) in our rheometers, to try to establish a correlation between rheology of pure components and dispersion morphology in the molten state. Within this context, we are impelled to assert that, despite the solubility parameter values, under our mixing conditions the polymer/bitumen interaction decreases as: HDPE/bitumen > HDPE-EPDM/bitumen > LDPEEPDM/bitumen > EPDM/bitumen > LDPE/bitumen, as indicated by η* and G′ results of Figures 7 and 8. It is important to note that these results reflect the response in the range 30-70 °C, which according to the interpretation given to our DMTA results (Figure 5), would correspond to a complex system composed by: maltene liquid phase (polar aromatics, naphthene aromatics, and saturates), polymer particles swollen by saturate resins, and free asphaltene particles surrounded by a liquid resin. At low temperatures, i.e., below the maximum observed in Figure 5 but above Tg of maltene, the picture would be different, since asphaltene particles would associate to form a complex structure. This difference in asphaltene status should be reflected in dynamic viscoelastic results. In Figure 10 we note that frequency dependence of G′ and G′′ decreases as temperature is lowered, evolving to G′ > G′′ at T ) 10 °C. This resembles, at a certain extent, the evolution of viscoelastic moduli near sol-gel transition, as has been studied by Winter.36,37 However, the rheological definition of a gel,38 meaning that the elastic modulus should tend to a certain value when frequency goes to zero, is not fulfilled at our lowest measuring temperature. This result favors the hypothesis of a compact structure, rather than a properly defined gel, at low temperatures. Another important conclusion of the results obtained (32) Hagen, A. P.; Jones, R.; Hofener, R. M.; Rabdolph, B. B.; Johnson, M. P. Proc. Assoc. Asphalt Paving Technol. 1984, 53, 119137 (33) Van Krevelen, D. W. Properties of Polymers; Elsevier: Amsterdam, 1990. (34) Polymer Handbook, 3rd ed.; Brandrup, J., Immergut, E. H., Eds.; Wiley: New York, 1989. (35) Pe´rez-Lepe, A.; Martı´nez-Boza, F.; Gallegos, C.; Gonza´lez, O.; Mun˜oz, M. E.; Santamarı´a, A. Paper in progress. (36) Winter, H. H.; Mours, M. Adv. Polym. Sci. 1997, 134, 165234. (37) Winter, H. H. Polym. Eng. Sci. 1987, 27 (22), 1698-1702. (38) Guenet, J. M. Thermoreversible Gelation of Polymers and Biopolymers; Academic: London, 1992.

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Figure 12. Complex viscosity as a function of frequency at T ) 10 °C for pure bitumen and polymer/bitumen blends.

Figure 10. Storage modulus (filled symbols) and loss modulus (empty symbols) as a function of frequency at three characteristic temperatures: (Ο) 10 °C, (0) 40 °C, (∆) 70 °C. (a) refers to pure bitumen and (b) to EPDM-HDPE/bitumen blend. The rest of the blends show the same trends.

effect of polymer addition is much lower at low temperatures: as most, viscosity and modulus are multiplied by 4, face to 25 at high temperatures. We have not yet a conclusive explanation for these results, but the hypothesis that on cooling a reduction of the effective volume of polymer particles takes place may be envisaged. The effect of temperature on viscoelastic results is also affected by two key factors: (a) The aforementioned state of asphaltenes, which are able to form a compact structure at low temperatures, but disperse as free particles at high temperatures, and (b) The parameter λ ) G*-polymer/G*-bitumen (that depends on temperature and frequency) which gives rise to an increase (when λ > 1) or a decrease (when λ < 1) of viscoelastic functions26 with respect to pure bitumen. This factor is not qualitatively relevant for HDPE-, LDPE-, HDPE-EPDM-, and LDPE-EPDM-based blends, since λ > 1 at low and high temperatures. However, for the EPDM/bitumen blend it is observed that λ < 1 from -30 to 30 °C, whereas above 30 °C, λ > 1: needless to say that this is probably crucial data to explain the different behavior shown by this blend in Figures 11 and 12 with respect to Figures 7 and 8. Concluding Remarks

Figure 11. Storage modulus as a function of frequency at T ) 10 °C for pure bitumen and polymer/bitumen blends.

at 10 °C is that the interdependence between viscoelastic results and polymer/bitumen interaction (the higher the viscosity, the stronger the interaction) is questionable. In fact, storage moduli and complex viscosities obtained at 10 °C (shown in Figures 11 and 12) give a ranking which differs from that obtained in the range 30-70 °C. In particular, this difference is evident in EPDM/bitumen blend, which does not show enhanced viscoelasticity at low temperature. Furthermore, the

‚The broad relaxation, observed in DMTA experiments at approximately 30 °C, is associated with the collapse of a compact structure, constituted by asphaltene particles surrounded by solid resin. Under our experimental conditions, the addition of polymer to bitumen does not modify this relaxation, probably because polymer and asphaltene interact, respectively, with different resins of the maltene phase. ‚Dynamic viscoelastic results in the range 30-70 (above relaxation peak) indicate a viscous response G′′ > G′ for all the samples. Both, elastic modulus and complex viscosity are enormously enhanced when polymer is added, especially in the case of HDPE and HDPE/ EPDM blend. The lack of compatibility between HDPE and EPDM and a certain interaction between LDPE and EPDM is reflected in the results of HPDE-EPDM/ bitumen and LDPE-EPDM/bitumen blends. The dynamic functions decrease as: HDPE/bitumen > HDPEEPDM/bitumen > LDPE-EPDM/bitumen > EPDM/ bitumen > LDPE/bitumen. We assume that the degree

Analyzing Polymer-Bitumen Interactions

of polymer-bitumen interaction, that is to say saturates absorption, follows the same order at least in this temperature range. This is in apparent contradiction to the results one could expect from solubility considerations, but we claim that rheological mixing conditions, leading in each case to different states of dispersion, can change polymer-bitumen interaction. ‚Dynamic viscoelastic results obtained below the relaxation peak, point out an elastic behavior (G′ > G′′) for all the samples, although maltene phase is well above its glass transition temperature. This is a symptom of the existence of a compact structure, albeit a gellike behavior is not envisaged. The effect of the addition of polymer is much less noticeable than at high temperatures. In particular, the addition of EPDM does

Energy & Fuels, Vol. 16, No. 5, 2002 1263

not enhance at all viscoelastic results of bitumen itself, probably because the complex modulus of the polymer is lower than that of bitumen at low temperatures. This makes it questionable to draw general conclusions about polymer-bitumen interaction, from the simple use of viscoelastic results obtained at a particular temperature. Acknowledgment. Financial support through MCYT (MAT99-0545-C02-02) (Spanish Government) and UPV/ EHU (00203.215-10107/1999) (University of The Basque Country) is acknowledged. EF020049L